Lead-free conductive paste composition and semiconductor devices made therewith

ABSTRACT

A lead-free conductive paste composition contains a source of an electrically conductive metal, a fusible material, an optional additive, and an organic vehicle. An article such as a high-efficiency photovoltaic cell is formed by a process of deposition of the lead-free paste composition on a semiconductor substrate (e.g., by screen printing) and firing the paste to remove the organic vehicle and sinter the metal and fusible material.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of co-pending U.S. patent applicationSer. No. 13/332,629, filed Dec. 21, 2011 and entitled “Lead-FreeConductive Paste Composition And Semiconductor Devices Made Therewith,”which application is incorporated herein in its entirety by referencethereto.

FIELD OF THE INVENTION

The present invention relates to a conductive paste composition that isuseful in the construction of a variety of electrical and electronicdevices, and more particularly to a lead-free paste composition usefulin creating conductive structures, including front-side electrodes forphotovoltaic devices.

TECHNICAL BACKGROUND OF THE INVENTION

A conventional photovoltaic cell incorporates a semiconductor structurewith a junction, such as a p-n junction formed with an n-typesemiconductor and a p-type semiconductor. For the typical p-baseconfiguration, a negative electrode is located on the side of the cellthat is to be exposed to a light source (the “front” side, which in thecase of a solar cell is the side exposed to sunlight), and a positiveelectrode is located on the other side of the cell (the “back” side).Radiation of an appropriate wavelength, such as sunlight, falling on thep-n junction serves as a source of external energy that generateselectron-hole pair charge carriers. These electron-hole pair chargecarriers migrate in the electric field generated by the p-n junction andare collected by electrodes on respective surfaces of the semiconductor.The cell is thus adapted to supply electric current to an electricalload connected to the electrodes, thereby providing electrical energyconverted from the incoming solar energy that can do useful work.Solar-powered photovoltaic systems are considered to be environmentallybeneficial in that they reduce the need for fossil fuels used inconventional electric power plants.

Industrial photovoltaic cells are commonly provided in the form of astructure, such as one based on a doped crystalline silicon wafer, thathas been metalized, i.e., provided with electrodes in the form ofelectrically conductive metal contacts through which the generatedcurrent can flow to an external electrical circuit load. Most commonly,these electrodes are provided on opposite sides of a generally planarcell structure. Conventionally, they are produced by applying suitableconductive metal pastes to the respective surfaces of the semiconductorbody and thereafter firing the pastes.

Photovoltaic cells are commonly fabricated with an insulating layer ontheir front side to afford an anti-reflective property that maximizesthe utilization of incident light. However, in this configuration, theinsulating layer normally must be removed to allow an overlaidfront-side electrode to make contact with the underlying semiconductorsurface. The front-side conductive metal paste typically includes aglass frit and a conductive species (e.g., silver particles) carried inan organic medium that functions as a vehicle. The electrode may beformed by depositing the paste composition in a suitable pattern (forinstance, by screen printing) and thereafter firing the pastecomposition and substrate to dissolve or otherwise penetrate theinsulating layer and sinter the metal powder, such that an electricalconnection with the semiconductor structure is formed.

The ability of the paste composition to penetrate the anti-reflectivecoating and form a strong bond with the substrate upon firing is highlydependent on the composition of the conductive paste and the firingconditions. Efficiency, a key measure of photovoltaic cell performance,is also influenced by the quality of the electrical contact made betweenthe fired conductive paste and the substrate.

Although various methods and compositions useful in forming devices suchas photovoltaic cells are known, there nevertheless remains a need forcompositions that permit fabrication of patterned conductive structuresthat provide improved overall device electrical performance and thatfacilitate the efficient manufacture of such devices.

SUMMARY OF THE INVENTION

An embodiment of the invention relates to a paste composition comprisingin admixture: (a) a source of electrically conductive metal; (b) afusible material comprising: 50-90 wt. % Bi₂O₃, 6.5-15 wt. % Al₂O₃, 2-26wt. % SiO₂, and 0-9 wt. % B₂O₃, the weight percentages being based onthe total fusible material, and wherein some or all of at least one ofthe oxides is optionally replaced by a fluoride of the same cation in anamount such that the fusible material comprises at most 5 wt. % offluorine, based on the total fusible material; and (c) an organicvehicle; and wherein the paste composition is lead-free.

Another aspect provides a process for forming an electrically conductivestructure on a substrate, the process comprising:

-   -   (a) providing a substrate having a first major surface;    -   (b) applying a paste composition onto a preselected portion of        the first major surface, wherein the paste composition comprises        in admixture:        -   i) a source of electrically conductive metal;        -   ii) at least one fusible material comprising:            -   50-90 wt. % Bi₂O₃,            -   6.5-15 wt. % Al₂O₃,            -   2-26 wt. % SiO₂, and            -   0-9 wt. % B₂O₃,            -   the weight percentages being based on the total fusible                material, and wherein some or all of at least one of the                oxides is optionally replaced by a fluoride of the same                cation in an amount such that the fusible material                comprises at most 5 wt. % of fluorine, based on the                total fusible material; and        -   iii) an organic vehicle,        -   wherein the paste composition is lead-free; and    -   (c) firing the substrate and paste composition thereon, whereby        the electrically conductive structure is formed on the        substrate.

Further, there is provided an article comprising a substrate and anelectrically conductive structure thereon, the article having beenformed by the foregoing process. Representative articles of this typeinclude a semiconductor device and a photovoltaic cell. In anembodiment, the substrate comprises a silicon wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood and further advantages willbecome apparent when reference is made to the following detaileddescription of the preferred embodiments of the invention and theaccompanying drawings, wherein like reference numerals denote similarelements throughout the several views and in which:

FIGS. 1A-1F depict successive steps of a process by which asemiconductor device may be fabricated. The device in turn may beincorporated into a photovoltaic cell. Reference numerals as used inFIGS. 1A-1F include the following:

-   -   10: p-type substrate    -   12: first major surface (front side) of substrate 10    -   14: second major surface (back side) of substrate 10    -   20: n-type diffusion layer    -   30: insulating layer    -   40: p+ layer    -   60: aluminum paste formed on back side    -   61: aluminum back electrode (obtained by firing back-side        aluminum paste)    -   70: silver or silver/aluminum paste formed on back side    -   71: silver or silver/aluminum back electrode (obtained by firing        back-side paste)    -   500: silver paste formed on front side according to the        invention    -   501: silver front electrode according to the invention (formed        by firing front-side silver paste)

DETAILED DESCRIPTION OF THE INVENTION

The present invention addresses the need for a process to manufacturehigh performance semiconductor devices having mechanically robust, highconductivity electrodes. The conductive paste composition providedherein is beneficially employed in the fabrication of front-sideelectrodes of photovoltaic devices. Ideally, a paste compositionpromotes the formation of a relatively low resistance contact betweenthe front-side metallization and the underlying semiconductor substrate.Suitable paste compositions are believed to aid in etching surfaceinsulating layers often employed in semiconductor structures such asphotovoltaic cells.

In an aspect, this invention provides a lead-free paste composition thatcomprises: a functional conductive component, such as a source ofelectrically conductive metal; a fusible material; and an organicvehicle.

The paste composition may comprise, in admixture, an inorganic solidsportion comprising (a) about 75% to about 99.5% by weight, or about 85to about 99% by weight, or about 95 to about 99% by weight, of a sourceof an electrically conductive metal; and (b) about 0.5% to about 10% byweight, or about 0.5% to about 8% by weight, or about 2% to about 8% byweight, or about 0.5 to about 5% by weight, or about 1 to about 3% byweight, of a fusible material, wherein the above stated contents ofconstituents (a) and (b) are based on the total weight of all theconstituents of the inorganic solids portion of the composition.

As further described below, the paste composition also comprises anorganic vehicle, which acts as a carrier for the inorganic constituents,which are dispersed therein. The paste composition may further includeadditional components such as surfactants, thickeners, thixotropes, andbinders.

Typically, electrodes and other conductive traces are provided by screenprinting the paste composition onto a substrate, although other forms ofprinting, such as plating, extrusion, inkjet, shaped or multipleprinting, or ribbons may also be used. After deposition, thecomposition, which typically comprises a conductive metal powder (e.g.,Ag) in an organic carrier, is fired at an elevated temperature.

The composition also can be used to form conductive traces, such asthose employed in a semiconductor module that is to be incorporated intoan electrical or electronic device. As would be recognized by a skilledartisan, the paste composition described herein can be termed“conductive,” meaning that the composition can be formed into astructure and thereafter processed to exhibit an electrical conductivitysufficient for conducting electrical current between devices orcircuitry connected thereto.

I. Inorganic Components

An embodiment of the present invention relates to a paste composition,which may include: an inorganic solids portion comprising a functionalmaterial providing electrical conductivity, a fusible material, and anorganic vehicle in which the inorganic solids are dispersed. The pastecomposition may further include additional components such assurfactants, thickeners, thixotropes, and binders.

A. Electrically Conductive Metal

The present paste composition includes a source of an electricallyconductive metal. Exemplary metals include without limitation silver,gold, copper, nickel, palladium, platinum, aluminum, and alloys andmixtures thereof. Silver is preferred for its processability and highconductivity.

The conductive metal may be incorporated directly in the present pastecomposition as a metal powder. In another embodiment, a mixture of twoor more such metals is directly incorporated. Alternatively, the metalis supplied by a metal oxide or salt that decomposes upon exposure tothe heat of firing to form the metal. As used herein, the term “silver”is to be understood as referring to elemental silver metal, alloys ofsilver, and mixtures thereof, and may further include silver derivedfrom silver oxide (Ag₂O or AgO) or silver salts such as AgCl, AgNO₃,AgOOCCH₃ (silver acetate), AgOOCF₃ (silver trifluoroacetate), Ag₃PO₄(silver orthophosphate), or mixtures thereof. Any other form ofconductive metal compatible with the other components of the pastecomposition also may be used.

Electrically conductive metal powder used in the present pastecomposition may be supplied as finely divided particles having any oneor more of the following morphologies: a powder form, a flake form, aspherical form, a granular form, a nodular form, a crystalline form,other irregular forms, or mixtures thereof. The electrically conductivemetal or source thereof may also be provided in a colloidal suspension,in which case the colloidal carrier would not be included in anycalculation of weight percentages of the solids of which the colloidalmaterial is part.

The particle size of the metal is not subject to any particularlimitation. As used herein, “particle size” is intended to refer to“median particle size” or d₅₀, by which is meant the 50% volumedistribution size. The distribution may also be characterized by d₉₀,meaning that 90% by volume of the particles are smaller than d₉₀. Volumedistribution size may be determined by a number of methods understood byone of skill in the art, including but not limited to laser diffractionand dispersion methods employed by a Microtrac particle size analyzer(Montgomeryville, Pa). Dynamic light scattering, e.g., using a modelLA-910 particle size analyzer available commercially from HoribaInstruments Inc. (Irvine, Calif.), may also be used. In variousembodiments, the median particle size is greater than 0.2 μm and lessthan 10 μm, or the median particle size is greater than 0.4 μm and lessthan 5 μm, as measured using the Horiba LA-910 analyzer.

The electrically conductive metal may comprise any of a variety ofpercentages of the composition of the paste composition. To attain highconductivity in a finished conductive structure, it is generallypreferable to have the concentration of the electrically conductivemetal be as high as possible while maintaining other requiredcharacteristics of the paste composition that relate to eitherprocessing or final use. In an embodiment, the silver or otherelectrically conductive metal may comprise about 75% to about 99% byweight, or about 85 to about 99% by weight, or about 95 to about 99% byweight, of the inorganic solid components of the paste composition. Inanother embodiment, the solids portion of the paste composition mayinclude about 80 to about 90 wt. % silver particles and about 1 to about9 wt. % silver flakes. In an embodiment, the solids portion of the pastecomposition may include about 70 to about 90 wt. %. silver particles andabout 1 to about 9 wt. % silver flakes. In another embodiment, thesolids portion of the paste composition may include about 70 to about 90wt. % silver flakes and about 1 to about 9 wt. % of colloidal silver. Ina further embodiment, the solids portion of the paste composition mayinclude about 60 to about 90 wt. % of silver particles or silver flakesand about 0.1 to about 20 wt. % of colloidal silver.

The electrically conductive metal used herein, particularly when inpowder form, may be coated or uncoated; for example, it may be at leastpartially coated with a surfactant to facilitate processing. Suitablecoating surfactants include, for example, stearic acid, palmitic acid, asalt of stearate, a salt of palmitate, and mixtures thereof. Othersurfactants that also may be utilized include lauric acid, oleic acid,capric acid, myristic acid, linoleic acid, and mixtures thereof. Stillother surfactants that also may be utilized include polyethylene oxide,polyethylene glycol, benzotriazole, poly(ethylene glycol)acetic acid,and other similar organic molecules. Suitable counter-ions for use in acoating surfactant include without limitation hydrogen, ammonium,sodium, potassium, and mixtures thereof. When the electricallyconductive metal is silver, it may be coated, for example, with aphosphorus-containing compound.

In an embodiment, one or more surfactants may be included in the organicvehicle in addition to any surfactant included as a coating ofconductive metal powder used in the present paste composition.

As further described below, the electrically conductive metal can bedispersed in an organic vehicle that acts as a carrier for the metalphase and other constituents present in the formulation.

B. Fusible Material

The present paste composition includes a fusible material. The term“fusible,” as used herein, refers to the ability of a material to becomefluid upon heating, such as the heating employed in a firing operation.In some embodiments, the fusible material is composed of one or morefusible subcomponents. For example, the fusible material may comprise aglass material, or a mixture of two or more glass materials. Glassmaterial in the form of a fine powder, e.g., as the result of acomminution operation, is often termed “frit” and is readilyincorporated in the present paste composition.

As used herein, the term “glass” refers to a particulate form of solidoxide or oxyfluoride that is at least predominantly amorphous, meaningthat short-range atomic order is preserved in the immediate vicinity ofany selected atom, that is, in the first coordination shell, butdissipates at greater atomic-level distances (i.e., there is no longrange periodic order). Hence, the X-ray diffraction pattern of a fullyamorphous material exhibits broad diffuse peaks, and not thewell-defined, narrow peaks of a crystalline material. In the latter, theregular spacing of characteristic crystallographic planes give rise tothe peaks, whose position in reciprocal space is in accordance withBragg's law. A glass material also does not show a substantialcrystallization exotherm upon heating close to or above its glasstransition temperature or softening point, T_(g), which is defined asthe second transition point seen in a differential thermal analysis(DTA) scan). In an embodiment, the softening point of glass materialused in the present paste composition is in the range of 300 to 800° C.

It is also contemplated that some or all of the fusible material may becomposed of material that exhibits some degree of crystallinity. Forexample, in some embodiments, a plurality of oxides are melted togetherand quenched as set forth above, resulting in a material that ispartially amorphous and partially crystalline. As would be recognized bya skilled person, such a material would produce an X-ray diffractionpattern having narrow, crystalline peaks superimposed on a pattern withbroad amorphous peaks. Alternatively, one or more constituents, or evensubstantially all of the fusible material, may be predominantly or evensubstantially fully crystalline. In an embodiment, crystalline materialuseful in the fusible material of the present paste composition may havea melting point of at most 800° C.

The present fusible material is described herein as includingpercentages of certain components (also termed the elementalconstituency). Specifically, the composition may be specified bydenominating individual components that may be combined in the specifiedpercentages to form a starting material that subsequently is processed,e.g., as described herein, to form a glass or other fusible material.Such nomenclature is conventional to one of skill in the art. In otherwords, the composition contains certain components, and the percentagesof those components are expressed as percentages of the correspondingoxide or other forms. As recognized by one of ordinary skill in the artof glass chemistry, a certain portion of volatile species may bereleased during the process of making the fusible material. An exampleof a volatile species is oxygen. The skilled person would also recognizethat a fusible material composition specified in this manner mayalternatively be prepared by supplying the required anions and cationsin requisite amounts from different components that, when mixed, yieldthe same overall composition. For example, in various embodiments,phosphorus could be supplied either from P₂O₅ or alternatively from aphosphate of one of the cations of the composition.

Although oxygen is typically the predominant anion in the fusiblematerial of the present paste composition, some portion of the oxygenmay be replaced by fluorine to alter certain properties, such aschemical, thermal, or rheological properties of the glass that affectfiring. One of ordinary skill would recognize that embodiments whereinthe glass composition contains fluorine can be prepared using fluorideanions supplied from a simple fluoride or an oxyfluoride. For example,the desired fluorine content can be supplied by replacing some or all ofan oxide nominally specified in the composition with the correspondingfluoride of the same cation, such as by replacing some or all of theLi₂O, Na₂O, or Bi₂O₃ nominally included with the amount of LiF, NaF, orBiF₃ needed to attain the desired level of F content. Of course, therequisite amount of F can be derived by replacing the oxides of morethan one of the fusible material's cations if desired. Other fluoridesources could also be used, including sources such as ammonium fluoridethat would decompose during the heating in typical glass preparation toleave behind residual fluoride anions. Useful fluorides include, but arenot limited to, BiF₃, AlF₃, NaF, LiF, ZrF₄, TiF₄, and ZnF₂. Certain ofthe present paste compositions containing fluorides beneficially includeNa and Li cations. Specifically, it has been found that if fluorine ispresent, the concomitant presence of Na and/or Li enhances deviceproperties of photovoltaic cells having front-side electrodes made withsuch a paste composition. In an embodiment, the fusible material of thepaste composition comprises 0-2 wt. % Li₂O, 0-3 wt. % Na₂O, and 0.5-4wt. % F, with the proviso that the total amount of Li₂O and Na₂O is atleast 0.5 wt. %.

It is known to those skilled in the art that a fusible material such asone prepared by a melting technique as described herein may becharacterized by known analytical methods that include, but are notlimited to: Inductively Coupled Plasma-Emission Spectroscopy (ICPES),Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES), andthe like. In addition, the following exemplary techniques may be used:X-Ray Fluorescence spectroscopy (XRF), Nuclear Magnetic Resonancespectroscopy (NMR), Electron Paramagnetic Resonance spectroscopy (EPR),Mössbauer spectroscopy, Electron microprobe Energy DispersiveSpectroscopy (EDS), Electron microprobe Wavelength DispersiveSpectroscopy (WDS), and Cathodoluminescence (CL). A skilled person couldcalculate percentages of starting components that could be processed toyield a particular fusible material, based on results obtained with suchanalytical methods.

The fusible materials described herein, including the glass compositionslisted in Table I, are not limiting; it is contemplated that one ofordinary skill in the art of glass chemistry could make minorsubstitutions of additional ingredients and not substantially change thedesired properties of the fusible material composition, including itsinteraction with a substrate and any insulating layer thereon.

A median particle size of the fusible material in the presentcomposition may be in the range of about 0.5 to 10 μm, or about 0.8 to 5μm, or about 1 to 3 μm, as measured using the Horiba LA-910 analyzer.

In an embodiment, the amount of fusible material in the pastecomposition may be in the range of 0.5 to 10 wt. %, 1 to 6 wt. %, 1 to 4wt. %, 1.5 to 2.5 wt. %, or 2 to 5 wt. % of the paste composition. Invarious embodiments, the fusible material of the present pastecomposition comprises: (a) 50-90 wt. % Bi₂O₃, 6.5-15 wt. % Al₂O₃, 2-26wt. % SiO₂, and 0-9 wt. % B₂O₃; or (b) 60-90 wt. % Bi₂O₃, 6.5-12.5 wt. %Al₂O₃, 2-19 wt. % SiO₂, and 0.5-7 wt. % B₂O₃, or (c) 70-90 wt. % Bi₂O₃,6.5-10 wt. % Al₂O₃, 2-12 wt. % SiO₂, and 1-5 wt. % B₂O₃, wherein theweight percentages are based on the total fusible material. In either ofthese compositions, some or all of at least one of the oxides isoptionally replaced by a fluoride of the same cation in an amount suchthat the fusible material comprises at most 5 wt. % of fluorine, basedon the total fusible material. Either composition may also furthercomprise at least one of: 0-6 wt. % ZnO, 0-3 wt. % P₂O₅, 0-5 wt. % Li₂O,0-5 wt. % Na₂O, 0-2 wt. % TiO₂, or 0-2 wt. % ZrO₂. In anotherembodiment, the fusible material consists essentially of 71-90 wt. %Bi₂O₃, 6.5-11 wt. % Al₂O₃, 2-13 wt. % SiO₂, and 1-5 wt. % B₂O₃; or 80-90wt. % Bi₂O₃, 6.5-8 wt. % Al₂O₃, 2-8 wt. % SiO₂, and 1.5-4 wt. % B₂O₃.

In an embodiment, the fusible material may be produced by conventionalglass making techniques and equipment. For the examples provided herein,the ingredients were weighed and mixed in the desired proportions andheated in a platinum alloy crucible in a furnace. The ingredients may beheated to a peak temperature (e.g., 800° C. to 1400° C., or 1000° C. to1200° C.) and held for a time such that the material forms a melt thatis substantially liquid and homogeneous (e.g., 20 minutes to 2 hours).The melt optionally is stirred, either intermittently or continuously.In an embodiment, the melting process results in a material wherein theconstituent chemical elements are fully mixed at an atomic level. Themolten material is then typically quenched in any suitable wayincluding, without limitation, passing it between counter-rotatingstainless steel rollers to form 0.25 to 0.50 mm thick platelets, bypouring it onto a thick stainless steel plate, or by pouring it intowater or other quench fluid. The resulting particles are then milled toform a powder or frit.

Other production techniques may also be used for the present fusiblematerial. One skilled in the art of producing such materials mighttherefore employ alternative synthesis techniques including, but notlimited to, melting in non-precious metal crucibles, melting in ceramiccrucibles, sol-gel, spray pyrolysis, or others appropriate for makingpowder forms of glass.

A skilled person would recognize that the choice of raw materials couldunintentionally include impurities that may be incorporated into thefusible material during processing. For example, the impurities may bepresent in the range of hundreds to thousands of parts per million.Impurities commonly occurring in industrial materials used herein areknown to one of ordinary skill.

The presence of the impurities would not substantially alter theproperties of the fusible material itself, paste compositions made withthe fusible material, or a fired device manufactured using the pastecomposition. For example, a solar cell employing a conductive structuremade using the present paste composition may have the efficiencydescribed herein, even if the composition includes impurities.

The fusible material used in the present composition is believed toassist in the partial or complete penetration of the oxide or nitrideinsulating layer commonly present on a silicon semiconductor waferduring firing. As described herein, this at least partial penetrationmay facilitate the formation of an effective, mechanically robustelectrical contact between a conductive structure manufactured using thepresent composition and the underlying silicon semiconductor of aphotovoltaic device structure.

In an embodiment, the present paste composition (including the fusiblematerial contained therein) is lead-free. As used in the presentspecification and the subjoined claims, the term “lead-free” refers to acomposition to which no lead has been specifically added (either aselemental lead or as a lead-containing alloy, compound, or other likesubstance), and in which the amount of lead present as a trace componentor impurity is 1000 parts per million (ppm) or less. In someembodiments, the amount of lead present as a trace component or impurityis less than 500 parts per million (ppm), or less than 300 ppm, or lessthan 100 ppm. The minimization of lead in the present paste compositionfacilitates the disposal or recycling of devices constructed with thecomposition and mitigates the health hazard associated with the knowntoxicity of lead-bearing substances, such as the present composition.Surprisingly and unexpectedly, photovoltaic cells exhibiting desirableelectrical properties, such as high conversion efficiency, are obtainedin some embodiments of the present disclosure, notwithstanding previousbelief in the art that substantial amounts of lead must be included in apaste composition to attain these levels.

In still another embodiment, the present paste composition is zinc-free.As used in the present specification and the subjoined claims, the term“zinc-free” refers to a composition to which no zinc has beenspecifically added (either as elemental zinc or as a zinc-containingalloy, compound, or other like substance), and in which the amount ofzinc present as a trace component or impurity is 1000 parts per million(ppm) or less. Although zinc is frequently incorporated in glasscompositions, in some instances the firing operation produces one ormore zinc silicate compositions that are non-conductive and believed tobe slightly deleterious to a photovoltaic cell's functional properties.In some embodiments, photovoltaic devices in which the present lead-freepaste composition is used to form front-side electrodes attainelectrical properties that are equivalent to, or better than, those ofdevices made with conventional leaded pastes. Those properties can, insome cases, be obtained without the Zn-based additives heretoforebelieved essential for known lead-free paste compositions.

The fusible material in the present paste composition may optionallycomprise a plurality of separate fusible substances, such as one or morefrits, or a substantially crystalline material with additional fritmaterial. In an embodiment, a first fusible subcomponent is chosen forits capability to rapidly digest an insulating layer, such as thattypically present on the front surface of a photovoltaic cell; furtherthe first fusible subcomponent may have strong corrosive power and lowviscosity. A second fusible subcomponent is optionally included toslowly blend with the first fusible subcomponent to alter the chemicalactivity. Preferably, the composition is such that the insulating layeris partially removed but without attacking the underlying emitterdiffused region, which would shunt the device, were the corrosive actionto proceed unchecked. Such fusible materials may be characterized ashaving a viscosity sufficiently high to provide a stable manufacturingwindow to remove insulating layers without damage to the diffused p-njunction region of a semiconductor substrate. Ideally, the firingprocess results in a substantially complete removal of the insulatinglayer without further combination with the underlying Si substrate orthe formation of substantial amounts of non-conducting or poorlyconducting inclusions.

C. Additive (s)

In some embodiments, the present paste composition separately includesat least one discrete additive that acts in concert with other pasteconstituents. The additive may play any of a number of roles by whichthe properties of a device produced using the paste are enhanced and/orthe processing is facilitated. In an embodiment, the additive content ofthe present paste composition ranges from 0 to about 5 wt. %, based onthe total weight of the paste composition.

In some embodiments, it is believed that during firing, the discreteadditive acts in concert with the fusible material in the present pastecomposition to promote etching and rapid digestion of the insulatinglayer conventionally used on the front side of a photovoltaic cell.Efficient etching in turn permits the formation of a low resistance,front-side electrical contact between the metal(s) of the compositionand the underlying substrate.

Other additives may act to promote doping of an emitter layer in a solarcell, to flux the fusible material during firing or otherwise enhanceadhesion of the conductive structure, or to control the paste rheology,e.g., by acting as an inorganic thixotrope.

II. Organic Vehicle

The inorganic components of the present composition are typically mixedwith an organic vehicle to form a relatively viscous material referredto as a “paste” or an “ink” that has a consistency and rheology thatrender it suitable for printing processes, including without limitationscreen printing. The mixing is typically done with a mechanical system,and the constituents may be combined in any order, as long as they areuniformly dispersed and the final formulation has characteristics suchthat it can be successfully applied during end use.

A wide variety of inert viscous materials can be admixed in an organicmedium in the present composition including, without limitation, aninert, non-aqueous liquid that may or may not contain thickeners orstabilizers. By “inert” is meant a material that may be removed by afiring operation without leaving any substantial residue or otheradverse effect that is detrimental to final conductor line properties.

The proportions of organic vehicle and inorganic components in thepresent paste composition can vary in accordance with the method ofapplying the paste and the kind of organic vehicle used. In anembodiment, the present paste composition typically contains about 76 to95 wt. %, or 85 to 95 wt. %, of the inorganic components and about 5 to24 wt. %, or 5 to 15 wt. %, of the organic vehicle.

The organic vehicle typically provides a medium in which the inorganiccomponents are dispersible with a good degree of stability. Inparticular, the composition preferably has a stability compatible notonly with the requisite manufacturing, shipping, and storage, but alsowith conditions encountered during deposition, e.g. by a screen printingprocess. Ideally, the rheological properties of the vehicle are suchthat it lends good application properties to the composition, includingstable and uniform dispersion of solids, appropriate viscosity andthixotropy for screen printing, appropriate wettability of the pastesolids and the substrate on which printing will occur, a rapid dryingrate after deposition, and stable firing properties.

Substances useful in the formulation of the organic vehicle of thepresent paste composition include, without limitation, ones disclosed inU.S. Pat. No. 7,494,607 and International Patent Application PublicationNo. WO 2010/123967 A2, both of which are incorporated herein in theirentirety for all purposes, by reference thereto. The disclosedsubstances include ethylhydroxyethyl cellulose, wood rosin, mixtures ofethyl cellulose and phenolic resins, cellulose acetate, celluloseacetate butyrate, polymethacrylates of lower alcohols, monobutyl etherof ethylene glycol, monoacetate ester alcohols, and terpenes such asalpha -or beta-terpineol or mixtures thereof with other solvents such askerosene, dibutylphthalate, butyl carbitol, butyl carbitol acetate,hexylene glycol and high-boiling alcohols and alcohol esters.

Solvents useful in the organic vehicle include, without limitation,ester alcohols and terpenes such as alpha- or beta-terpineol or mixturesthereof with other solvents such as kerosene, dibutylphthalate, butylcarbitol, butyl carbitol acetate, hexylene glycol, and high-boilingalcohols and alcohol esters. A preferred ester alcohol is themonoisobutyrate of 2,2,4-trimethyl-1,3-pentanediol, which is availablecommercially from Eastman Chemical (Kingsport, Tenn.) as TEXANOL™. Someembodiments may also incorporate volatile liquids in the organic vehicleto promote rapid hardening after application on the substrate. Variouscombinations of these and other solvents are formulated to provide thedesired viscosity and volatility.

In an embodiment, the organic vehicle may include one or more componentsselected from the group consisting of: bis(2-(2butoxyethoxy)ethyl)adipate, dibasic esters, octyl epoxy tallate (DRAPEX® 4.4 from WitcoChemical), Oxocol (isotetradecanol made by Nissan Chemical) and FORALYN™110 (pentaerythritol ester of hydrogenated rosin from Eastman ChemicalBV). The paste compositions may also include additional additives orcomponents.

The dibasic ester useful in the present paste composition may compriseone or more dimethyl esters selected from the group consisting ofdimethyl ester of adipic acid, dimethyl ester of glutaric acid, anddimethyl ester of succinic acid. Various forms of such materialscontaining different proportions of the dimethyl esters are availableunder the DBE® trade name from Invista (Wilmington, Del.). For thepresent paste composition, a preferred version is sold as DBE-3 and issaid by the manufacturer to contain 85 to 95 weight percent dimethyladipate, 5 to 15 weight percent dimethyl glutarate, and 0 to 1.0 weightpercent dimethyl succinate based on total weight of dibasic ester.

Further ingredients optionally may be incorporated in the organicvehicle, such as thickeners, stabilizers, and/or other common additivesknown to those skilled in the art. The organic vehicle may be a solutionof one or more polymers in a solvent. Additionally, effective amounts ofadditives, such as surfactants or wetting agents, may be a part of theorganic vehicle. Such added surfactant may be included in the organicvehicle in addition to any surfactant included as a coating on theconductive metal powder of the paste composition. Suitable wettingagents include phosphate esters and soya lecithin. Both inorganic andorganic thixotropes may also be present.

Among the commonly used organic thixotropic agents are hydrogenatedcastor oil and derivatives thereof, but other suitable agents may beused instead of, or in addition to, these substances. It is, of course,not always necessary to incorporate a thixotropic agent since thesolvent and resin properties coupled with the shear thinning inherent inany suspension may alone be suitable in this regard.

A polymer frequently used in printable conductive metal pastes is ethylcellulose. Other exemplary polymers that may be used includeethylhydroxyethyl cellulose, wood rosin and derivatives thereof,mixtures of ethyl cellulose and phenolic resins, cellulose acetate,cellulose acetate butyrate, poly(methacrylate)s of lower alcohols, andmonoalkyl ethers of ethylene glycol monoacetate.

Any of these polymers may be dissolved in a suitable solvent, includingthose described herein.

The polymer in the organic vehicle may be present in the range of 0.1wt. % to 5 wt. % of the total composition. The present paste compositionmay be adjusted to a predetermined, screen-printable viscosity, e.g.,with additional solvent(s).

III. Formation of Conductive Structures

An aspect of the invention provides a process that may be used to form aconductive structure on a substrate. The process generally comprises thesteps of providing the substrate, applying a paste composition, andfiring the substrate. Ordinarily, the substrate is planar and relativelythin, thus defining first and second major surfaces on its oppositesides.

Application

The present composition can be applied as a paste onto a preselectedportion of a major surface of the substrate in a variety of differentconfigurations or patterns. The preselected portion may comprise anyfraction of the total first major surface area, including substantiallyall of the area. In an embodiment, the paste is applied on asemiconductor substrate, which may be single-crystal, multi-crystal,polycrystalline, or ribbon silicon, or any other semiconductor material.

The application can be accomplished by a variety of depositionprocesses, including printing. Exemplary deposition processes include,without limitation, plating, extrusion or co-extrusion, dispensing froma syringe, and screen, inkjet, shaped, multiple, and ribbon printing.The paste composition ordinarily is applied over any insulating layerpresent on the first major surface of the substrate.

The conductive composition may be printed in any useful pattern. Forexample, the electrode pattern used for the front side of a photovoltaiccell commonly includes a plurality of narrow grid lines or fingersconnected to one or more bus bars. In an embodiment, the width of thelines of the conductive fingers may be 20 to 200 μm; 40 to 150 μm; or 60to 100 μm. In an embodiment, the thickness of the lines of theconductive fingers may be 5 to 50 μm; 10 to 35 μm; or 15 to 30 μm. Sucha pattern permits the generated current to be extracted without undueresistive loss, while minimizing the area of the front side obscured bythe metallization, which reduces the amount of incoming light energythat can be converted to electrical energy. Ideally, the features of theelectrode pattern should be well defined, with a preselected thicknessand shape, and have high electrical conductivity and low contactresistance with the underlying structure.

Conductors formed by printing and firing a paste such as that providedherein are often denominated as “thick film” conductors, since they areordinarily substantially thicker than traces formed by atomisticprocesses, such as those used in fabricating integrated circuits. Forexample, thick film conductors may have a thickness after firing ofabout 1 to 100 μm. Consequently, paste compositions that in theirprocessed form provide conductivity and are suitably applied usingprinting processes are often called “thick film pastes” or “conductiveinks.”

Firing

A firing operation may be used in the present process to effect asubstantially complete burnout of the organic vehicle from the depositedpaste. The firing typically involves volatilization and/or pyrolysis ofthe organic materials. A drying operation optionally precedes the firingoperation, and is carried out at a modest temperature to harden thepaste composition by removing its most volatile organics.

The firing process is believed to remove the organic vehicle, sinter theconductive metal in the composition, and establish electrical contactbetween the semiconductor substrate and the fired conductive metal.Firing may be performed in an atmosphere composed of air, nitrogen, aninert gas, or an oxygen-containing mixture such as a mixed gas of oxygenand nitrogen.

In one embodiment, the temperature for the firing may be in the rangebetween about 300° C. to about 1000° C., or about 300° C. to about 525°C., or about 300° C. to about 650° C., or about 650° C. to about 1000°C. The firing may be conducted using any suitable heat source. In anembodiment, the firing is accomplished by passing the substrate bearingthe printed paste composition pattern through a belt furnace at hightransport rates, for example between about 100 to about 500 cm perminute, with resulting hold-up times between about 0.05 to about 5minutes. Multiple temperature zones may be used to control the desiredthermal profile, and the number of zones may vary, for example, between3 to 11 zones. The temperature of a firing operation conducted using abelt furnace is conventionally specified by the furnace set point in thehottest zone of the furnace, but it is known that the peak temperatureattained by the passing substrate in such a process is somewhat lowerthan the highest set point. Other batch and continuous rapid firefurnace designs known to one of skill in the art are also contemplated.

The fired electrode may include components, compositions, and the like,resulting from the firing and sintering process. For example, inembodiments wherein the paste composition includes Zn-containingsubstances, the fired electrode may include zinc silicates, includingbut not limited to, willemite (Zn₂SiO₄) and Zn_(1.7)SiO_(4-x) (in anembodiment, x may be 0.3). In a further embodiment the fired electrodemay include bismuth silicates, including but not limited to Bi₄(SiO₄)₃or Bi₂SiO₅.

In a further embodiment, other conductive and device enhancing materialsare applied prior to firing to the opposite type region of thesemiconductor device. The various materials may be applied and thenco-fired, or they may be applied and fired sequentially.

In an embodiment, the opposite type region may be on the non-illuminated(back) side of the device, i.e., its second major surface. The materialsserve as electrical contacts, passivating layers, and solderable tabbingareas. In an aspect of this embodiment, the back-side conductivematerial may contain aluminum. Exemplary back-side aluminum-containingcompositions and methods of application are described, for example, inUS 2006/0272700, which is hereby incorporated herein in its entirety forall purposes by reference thereto. Suitable solderable tabbing materialsinclude those containing aluminum and silver. Exemplary tabbingcompositions containing aluminum and silver are described, for examplein US 2006/0231803, which is hereby incorporated herein in its entiretyfor all purposes by reference thereto.

In a further embodiment, the present paste composition may be employedin the construction of semiconductor devices wherein the p and n regionsare formed side-by-side in a substrate, instead of being respectivelyadjacent to opposite major surfaces of the substrate. In animplementation in this configuration, the electrode-forming materialsmay be applied in different portions of a single side of the substrate,e.g., on the non-illuminated (back) side of the device, therebymaximizing the amount of light incident on the illuminated (front) side.

Insulating layer

In some embodiments of the invention, the paste composition is used inconjunction with a substrate, such as a semiconductor substrate, havingan insulating layer present on one or more of the substrate's majorsurfaces. The layer may comprise one or more components selected fromaluminum oxide, titanium oxide, silicon nitride, SiN_(x):H (siliconnitride containing hydrogen for passivation during subsequent firingprocessing), silicon oxide, and silicon oxide/titanium oxide, and may bein the form of a single, homogeneous layer or multiple sequentialsub-layers of any of these materials. Silicon nitride and SiN_(x):H arewidely used.

The insulating layer provides some embodiments of the cell with ananti-reflective property, which lowers the cell's surface reflectance oflight incident thereon, thereby improving the cell's utilization of theincident light and increasing the electrical current it can generate.Thus, the insulating layer is often denoted as an anti-reflectivecoating (ARC). The thickness of the layer preferably is chosen tomaximize the anti-reflective property in accordance with the layermaterial's composition and refractive index. In one approach, thedeposition processing conditions are adjusted to vary the stoichiometryof the layer, thereby altering properties such as the refractive indexto a desired value. For a silicon nitride layer with a refractive indexof about 1.9 to 2.0, a thickness of about 700 to 900 Å (70 to 90 nm) issuitable.

The insulating layer may be deposited on the substrate by methods knownin the microelectronics art, such as any form of chemical vapordeposition (CVD) including plasma-enhanced CVD (PECVD) and thermal CVD,thermal oxidation, or sputtering. In another embodiment, the substrateis coated with a liquid material that under thermal treatment decomposesor reacts with the substrate to form the insulating layer. In stillanother embodiment, the substrate is thermally treated in the presenceof an oxygen-or nitrogen-containing atmosphere to form an insulatinglayer. Alternatively, no insulating layer is specifically applied to thesubstrate, but a naturally forming substance, such as silicon oxide on asilicon wafer, may function as an insulating layer.

The present method optionally includes the step of forming theinsulating layer on the semiconductor substrate prior to the applicationof the paste composition.

In some implementations of the present process, the paste composition isapplied over any insulating layer present on the substrate, whetherspecifically applied or naturally occurring. The paste's fusiblematerial and any additive present may act in concert to combine with,dissolve, or otherwise penetrate some or all of the thickness of anyinsulating layer material during firing. Preferably, good electricalcontact between the paste composition and the underlying semiconductorsubstrate is thereby established. Ideally, the firing results in asecure attachment of the conductive metal structure to the substrate,with a metallurgical bond being formed over substantially all the areaof the substrate covered by the conductive element. In an embodiment,the conductive metal is separated from the silicon by a nano-scale glasslayer (typically of order 5 nm or less) through which the photoelectronstunnel. In another embodiment, contact is made between the conductivemetal and the silicon by a combination of direct metal-to-siliconcontact and tunneling through thin glass layers.

The mechanical quality of the attachment of the conductive metalstructure to the underlying substrate may be characterized using a pulltest. In one implementation, this pull test may be carried out asfollows. First, a backing plate made of ceramic or other suitablenon-deformable material may be glued to the back side of the substrateto provide stiffness and mechanical reinforcement. A soldering operationis then carried out to attach a solder ribbon to some portion of theconductive metal structure, such as the bus bar. A suitable flux may beapplied beforehand and then heat is supplied from the tip of aconventional soldering iron to melt a portion of the solder ribbon andmake the attachment. The strength of the bonding of the metallization tothe substrate is then determined from the force in a direction normal tothe substrate's major surface required to separate the ribbon from thesubstrate. Such testing is conveniently carried out using a mechanicaltesting machine, such as one made by the Instron Company (Norwood,Mass.).

Firing also promotes the formation of both good electrical conductivityin the conductive element itself and a low-resistance connection to thesubstrate, e.g., by sintering the conductive metal particles and etchingthrough the insulating layer. While some embodiments may function withelectrical contact that is limited to conductive domains dispersed overthe printed area, it is preferred that the contact be uniform oversubstantially the entire printed area.

Structures

An embodiment of the present invention relates to a structure comprisinga substrate and a conductive electrode, which may be formed by theprocess described above.

Semiconductor Device Manufacture

The structures described herein may be useful in the manufacture ofsemiconductor devices, including photovoltaic devices. An embodiment ofthe invention relates to a semiconductor device containing one or morestructures described herein. Another embodiment relates to aphotovoltaic device containing one or more structures described herein.Still further, there is provided a photovoltaic cell containing one ormore structures described herein and a solar panel containing one ormore of these structures.

In another aspect, the present invention relates to a device, such as anelectrical, electronic, semiconductor, photodiode, or photovoltaicdevice. Various embodiments of the device include a junction-bearingsemiconductor substrate and an insulating layer, such as a siliconnitride layer, present on a first major surface of the substrate.

One possible sequence of steps implementing the present process formanufacture of a photovoltaic cell device is depicted by FIGS. 1A-1F.

FIG. 1A shows a p-type substrate 10, which may be single-crystal,multicrystalline, or polycrystalline silicon. For example, substrate 10may be obtained by slicing a thin layer from an ingot that has beenformed from a pulling or casting process. Surface damage andcontamination (from slicing with a wire saw, for example) may be removedby etching away about 10 to 20 μm of the substrate surface using anaqueous alkali solution such as aqueous potassium hydroxide or aqueoussodium hydroxide, or using a mixture of hydrofluoric acid and nitricacid. In addition, the substrate may be washed with a mixture ofhydrochloric acid and optional hydrogen peroxide to remove heavy metalssuch as iron adhering to the substrate surface. Substrate 10 may have afirst major surface 12 that is textured to reduce light reflection.Texturing may be produced by etching a major surface with an aqueousalkali solution such as aqueous potassium hydroxide or aqueous sodiumhydroxide. Substrate 10 may also be formed from a silicon ribbon.

In FIG. 1B, an n-type diffusion layer 20 is formed to create a p-njunction with p-type material below. The n-type diffusion layer 20 canbe formed by any suitable doping process, such as thermal diffusion ofphosphorus (P) provided from phosphorus oxychloride (POCl₃). In theabsence of any particular modifications, the n-type diffusion layer 20is formed over the entire surface of the silicon p-type substrate. Thedepth of the diffusion layer can be varied by controlling the diffusiontemperature and time, and is generally formed in a thickness range ofabout 0.3 to 0.5 μm. The n-type diffusion layer may have a sheetresistivity from several tens of ohms per square up to about 120 ohmsper square.

After protecting one surface of the n-type diffusion layer 20 with aresist or the like, the n-type diffusion layer 20 is removed from mostsurfaces by etching so that it remains only on the first major surface12 of substrate 10, as shown in FIG. 1C. The resist is then removedusing an organic solvent or the like.

Next, as shown in FIG. 1D, an insulating layer 30, which also functionsas an anti-reflective coating, is formed on the n-type diffusion layer20. The insulating layer is commonly silicon nitride, but can also be alayer of another material, such as SiN_(x):H (i.e., the insulating layercomprises hydrogen for passivation during subsequent firing processing),titanium oxide, silicon oxide, mixed silicon oxide/titanium oxide, oraluminum oxide. The insulating layer can be in the form of a singlelayer or multiple layers of the same or different materials.

Next, electrodes are formed on both major surfaces 12 and 14 of thesubstrate. As shown in FIG. 1E, a paste composition 500 of thisinvention is screen printed on the insulating layer 30 of the firstmajor surface 12 and then dried. For a photovoltaic cell, pastecomposition 500 is typically applied in a predetermined pattern ofconductive lines extending from one or more bus bars that occupy apredetermined portion of the surface. In addition, aluminum paste 60 andback-side silver paste 70 are screen printed onto the back side (thesecond major surface 14 of the substrate) and successively dried. Thescreen printing operations may be carried out in any order. For the sakeof production efficiency, all these pastes are typically processed byco-firing them at a temperature in the range of about 700° C. to about975° C. for a period of from several seconds to several tens of minutesin air or an oxygen-containing atmosphere. An infrared-heated beltfurnace is conveniently used for high throughput.

As shown in FIG. 1F, the firing causes the depicted paste composition500 on the front side to sinter and penetrate through the insulatinglayer 30, thereby achieving electrical contact with the n-type diffusionlayer 20, a condition known as “fire through.” This fired-through state,i.e., the extent to which the paste reacts with and passes through theinsulating layer 30, depends on the quality and thickness of theinsulating layer 30, the composition of the paste, and on the firingconditions. A high-quality fired-through state is believed to be animportant factor in obtaining high conversion efficiency in aphotovoltaic cell. Firing thus converts paste 500 into electrode 501, asshown in FIG. 1F.

The firing further causes aluminum to diffuse from the back-sidealuminum paste into the silicon substrate, thereby forming a p+ layer40, containing a high concentration of aluminum dopant. This layer isgenerally called the back surface field (BSF) layer, and helps toimprove the energy conversion efficiency of the solar cell. Firingconverts the dried aluminum paste 60 to an aluminum back electrode 61.The back-side silver paste 70 is fired at the same time, becoming asilver or silver/aluminum back electrode 71. During firing, the boundarybetween the back-side aluminum and the back-side silver assumes thestate of an alloy, thereby achieving electrical connection. Most areasof the back electrode are occupied by the aluminum electrode, owing inpart to the need to form a p+ layer 40. Since there is no need forincoming light to penetrate the back side, substantially the entiresurface may be covered. At the same time, because soldering to analuminum electrode is unfeasible, a silver or silver/aluminum backelectrode is formed on limited areas of the back side as an electrode topermit soldered attachment of interconnecting copper ribbons or thelike.

While the present invention is not limited by any particular theory ofoperation, it is believed that, upon firing, the fusible material, withany additive component present acting in concert, promotes etching andrapid digestion of the insulating layer conventionally used on the frontside of a photovoltaic cell. Efficient etching in turn permits theformation of a low resistance, front-side electrical contact between themetal(s) of the composition and the underlying substrate.

It will be understood that the present paste composition and process mayalso be used to form electrodes, including a front-side electrode, of aphotovoltaic cell in which the p- and n-type layers are reversed fromthe construction shown in FIGS. 1A-1F, so that the substrate is n-typeand a p-type material is formed on the front side.

In yet another embodiment, this invention provides a semiconductordevice that comprises a semiconductor substrate having a first majorsurface; an insulating layer optionally present on the first majorsurface of the substrate; and, disposed on the first major surface, aconductive electrode pattern having a preselected configuration andformed by firing a paste composition as described above.

A semiconductor device fabricated as described above may be incorporatedinto a photovoltaic cell. In another embodiment, this invention thusprovides a photovoltaic cell array that includes a plurality of thesemiconductor devices as described, and made as described, herein.

EXAMPLES

The operation and effects of certain embodiments of the presentinvention may be more fully appreciated from a series of examples(Examples 1-8), as described below. The embodiments on which theseexamples are based are representative only, and the selection of thoseembodiments to illustrate aspects of the invention does not indicatethat materials, components, reactants, conditions, techniques and/orconfigurations not described in the examples are not suitable for useherein, or that subject matter not described in the examples is excludedfrom the scope of the appended claims and equivalents thereof.

Examples 1-6 Paste Preparation

Each of the fusible material compositions set forth in Table I wasprepared by melting the requisite ingredients in a covered Pt cruciblethat was heated in air from room temperature to 1100° C. (Examples 1-3and 6) or 1200° C. (Examples 4-5) over a period of 1.5 hours, and heldat the respective temperature for 1 hour. Each melt was separatelypoured onto the flat surface of a cylindrically-shaped stainless steelblock (8 cm high, 10 cm in diameter). The cooled, yellow-oramber-colored buttons were pulverized to −100 mesh frit.

Then the frit was ball milled in a polyethylene container with zirconiamedia and a suitable liquid, such as water, isopropyl alcohol, or watercontaining 0.5 wt. % TRITON™ X-100 octylphenol ethoxylate surfactant(available from Dow Chemical Company, Midland, Mich.) until the d₅₀ wasin the range of 0.5 to 0.7 μm.

TABLE I Fusible Material Compositions Composition (wt. %) Example Bi₂O₃SiO₂ B₂O₃ Al₂O₃ Li₂O Na₂O BiF₃ 1 84.08 6.54 2.88 6.50 0 0 0 2 82.73 6.442.84 8.00 0 0 0 3 80.92 6.30 2.78 10.00 0 0 0 4 78.68 6.12 2.70 12.50 00 0 5 76.43 5.95 2.62 15.00 0 0 0 6 79.68 3.68 3.41 9.03 0.57 1.19 2.43

In accordance with an aspect of the invention, the fusible materials ofExamples 1 to 6 were formulated in paste compositions suitable forscreen printing. The pastes, before adjusting their viscosities withadditional solvent, consisted of approximately 9.7 wt. % vehicle and 2to 5 wt. % fusible material, with the remainder being silver powder.Examples 1 to 5 used 2 wt. % fusible material in the paste, and Example6 used 5 wt. %.

The organic vehicle was prepared as a master batch using a Thinky mixer(available from Thinky USA, Inc., Laguna Hills, Calif.) to mix theingredients listed in Table II below, with percentages given by weight.

TABLE II Organic Vehicle Composition Ingredient Trade Name wt. % 11%ethyl cellulose (50-52% ethoxyl) 13.98% dissolved in TEXANOL ™ solvent8% ethyl cellulose (48-50% ethoxyl) 5.38% dissolved in TEXANOL ™ solventtallowpropylenediamine dioleate DUOMEEN ® TDO 10.75% pentaerythritolester of hydrogenated FORALYN ™ 110 26.88% rosin* Hydrogenated castoroil derivative THIXATROL ® ST 5.38% Dibasic ester DBE-3 37.63%

The organic materials used were: ethyl cellulose (Ashland Chemical Co.,Covington, Ky.); DUOMEEN® TDO (Akzo Nobel Surface Chemistry, Chicago,Ill.); FORALYN™ 110 (Eastman Chemical Co., Kingsport, Tenn.); THIXATROL®ST (Elementis, Hightstown, N.J.); DBE-3 (Invista, Wilmington, Del.); andTEXANOL™ ester alcohol (Eastman Chemical Co., Kingsport, Tenn.). Twogrades of ethyl cellulose having different viscosities, resulting fromdifferent average molecular weights, were combined. The first grade wasspecified by the manufacturer as having a nominal viscosity of about 80to 105 Pa-s, and the second grade had nominal viscosity of about 8 to 11Pa-s, with each viscosity determined as that of a 5% solution of theethyl cellulose in an 80/20 mixture of toluene/ethanol. The amount ofeach grade and the TEXANOL™ solvent dilution used were adjusted toobtain a vehicle ultimately affording a viscosity suitable for screenprinting. In each case, a suitable small portion of TEXANOL™ was addedat the end to adjust the final viscosity to a level permitting thecomposition to be screen printed onto a substrate. Typically, a finalpaste composition having a viscosity of about 300 Pa-s was found toyield good screen printing results, but some variation, for example ±50Pa-s or more would be acceptable, depending on the precise printingparameters.

Silver powder, represented by the manufacturer as having a predominantlyspherical shape and having a particle size distribution with a d₉₀ of3.8 μm as measured in an isopropyl alcohol dispersion using a HoribaLA-910 analyzer, was combined with the milled frit in a glass jar andtumble mixed for 15 minutes. The inorganic mixture was then added bythirds to a Thinky jar containing the organic ingredients andThinky-mixed for 1 minute at 2000 RPM after each addition. The paste wascooled and the viscosity was adjusted to between about 300 and 400 Pa-sby adding solvent and Thinky mixing for 1 minute at 2000 RPM. This stepwas repeated until the correct viscosity was achieved. The paste wasthen milled on a three-roll mill (Charles Ross and Son, Hauppauge, N.Y.)with a 25 μm gap for 3 passes at zero pressure and 3 passes at 100 psi(689 kPa).

The degree of dispersion of each paste composition was measured usingcommercial fineness of grind (FOG) gauges (Precision Gage and Tool,Dayton, Ohio) in accordance with ASTM Standard Test Method D 1210-05,which is promulgated by ASTM International, West Conshohocken, Pa., andis incorporated herein by reference. The FOG values in Table III arespecified as X/Y, meaning that the size of the largest particle detectedis X μm and the median size is Y μm.

TABLE III FOG Values of Paste Compositions FOG Value Example (μm/μm) 19/3 2 2/1 3 7/3 4 2/0 5 2/1 6 4/2

Each paste composition was allowed to sit for 24 hours, then itsviscosity was adjusted to be between 175 and 300 Pa-s, to render itsuitable for screen printing. The viscometer was a Brookfield viscometer(Brookfield Inc., Middleboro, Mass.) with a #14 spindle and a #6 cup.Viscosity values were taken after 3 minutes at 10 RPM.

Example 7 Photovoltaic Cell Fabrication

Photovoltaic cells in accordance with an aspect of the invention weremade using the paste compositions of Examples 1 to 6 to form thefront-side electrodes.

For convenience, the fabrication and electrical testing were carried outusing 28 mm×28 mm “cut down” wafers prepared by dicing 156 mm×156 mmstarting wafers using a diamond wafering saw. The test wafers (DeutscheCell, 200 μm thick, 65 ohms per square) were screen printed using anAMI-Presco (AMI, North Branch, N.J.) MSP-485 screen printer, first toform a full ground plane back-side conductor using a conventionalAl-containing paste, SOLAMET® PV381 (available from DuPont, Wilmington,Del.), and thereafter to form a bus bar and eleven conductor lines at a0.254 cm pitch on the front surface using the various exemplary pastecompositions herein. After printing and drying, cells were fired in aBTU rapid thermal processing, multi-zone belt furnace (BTUInternational, North Billerica, Mass.). The firing temperatures reportedin the examples are the furnace set point temperatures for the hottestfurnace zone. This temperature was found to be approximately 125° C.greater than the wafer temperature actually attained during the cell'spassage through the furnace. 25 cells were printed using each paste; 5cells were fired at each setpoint temperature in a 5-temperature ladder.After firing, the median conductor line width was 120 μm and the meanline height was 15 μm. The bus bar was 1.25 mm wide. The median lineresistivity was 3.0 μΩ-cm. Performance of “cut-down” 28 mm×28 mm cellsis known to be impacted by edge effects which reduce the overallphotovoltaic cell efficiency by ˜5% from what would be obtained withfull-size wafers.

Example 8 Photovoltaic Cell Testing

Electrical properties of photovoltaic cells fabricated as set forth inExample 6 were measured at 25±1.0° C. using an ST-1000 IV tester(Telecom STV Co., Moscow, Russia). The Xe arc lamp in the IV testersimulated sunlight with a known intensity and irradiated the frontsurface of the cell. The tester used a four contact method to measurecurrent (I) and voltage (V) at approximately 400 load resistancesettings to determine the cell's I-V curve. Efficiency was calculatedfrom the I-V curve for each cell. Mean and median efficiency wascalculated for the five cells of each test condition. This testingprotocol is exemplary. Other equipment and procedures for testingefficiencies will be recognized by one of ordinary skill in the art.

Data shown in Table IV indicate the highest measured mean efficiency forthe five cells of each test group, and the firing temperature at whicheach value was obtained.

TABLE IV Electrical Performance of Photovoltaic Cells Firing Mean Temp.Efficiency Example (° C.) (%) 1 930 14.55 2 925 15.00 3 955 15.31 4 95514.99 5 940 15.57 6 920 15.48

The data of Table IV demonstrate that the present high-alumina pastecomposition can be used to manufacture photocells having excellentelectrical properties, including high efficiency.

Having thus described the invention in rather full detail, it will beunderstood that this detail need not be strictly adhered to but thatfurther changes and modifications may suggest themselves to one skilledin the art, all falling within the scope of the invention as defined bythe subjoined claims

Where a range of numerical values is recited or established herein, therange includes the endpoints thereof and all the individual integers andfractions within the range, and also includes each of the narrowerranges therein formed by all the various possible combinations of thoseendpoints and internal integers and fractions to form subgroups of thelarger group of values within the stated range to the same extent as ifeach of those narrower ranges was explicitly recited. Where a range ofnumerical values is stated herein as being greater than a stated value,the range is nevertheless finite and is bounded on its upper end by avalue that is operable within the context of the invention as describedherein. Where a range of numerical values is stated herein as being lessthan a stated value, the range is nevertheless bounded on its lower endby a non-zero value.

In this specification, unless explicitly stated otherwise or indicatedto the contrary by the context of usage, where an embodiment of thesubject matter hereof is stated or described as comprising, including,containing, having, being composed of, or being constituted by or ofcertain features or elements, one or more features or elements inaddition to those explicitly stated or described may be present in theembodiment. An alternative embodiment of the subject matter hereof,however, may be stated or described as consisting essentially of certainfeatures or elements, in which embodiment features or elements thatwould materially alter the principle of operation or the distinguishingcharacteristics of the embodiment are not present therein. A furtheralternative embodiment of the subject matter hereof may be stated ordescribed as consisting of certain features or elements, in whichembodiment, or in insubstantial variations thereof, only the features orelements specifically stated or described are present. Additionally, theterm “comprising” is intended to include examples encompassed by theterms “consisting essentially of” and “consisting of.” Similarly, theterm “consisting essentially of” is intended to include examplesencompassed by the term “consisting of.”

When an amount, concentration, or other value or parameter is given aseither a range, preferred range, or a list of upper preferable valuesand lower preferable values, this is to be understood as specificallydisclosing all ranges formed from any pair of any upper range limit orpreferred value and any lower range limit or preferred value, regardlessof whether ranges are separately disclosed. Where a range of numericalvalues is recited herein, unless otherwise stated, the range is intendedto include the endpoints thereof, and all integers and fractions withinthe range. It is not intended that the scope of the invention be limitedto the specific values recited when defining a range

In this specification, unless explicitly stated otherwise or indicatedto the contrary by the context of usage,

(a) amounts, sizes, ranges, formulations, parameters, and otherquantities and characteristics recited herein, particularly whenmodified by the term “about”, may but need not be exact, and may also beapproximate and/or larger or smaller (as desired) than stated,reflecting tolerances, conversion factors, rounding off, measurementerror, and the like, as well as the inclusion within a stated value ofthose values outside it that have, within the context of this invention,functional and/or operable equivalence to the stated value; and

(b) all numerical quantities of parts, percentage, or ratio are given asparts, percentage, or ratio by weight; the stated parts, percentage, orratio by weight may or may not add up to 100.

What is claimed is:
 1. A process for forming an electrically conductivestructure on a substrate, the process comprising: (a) providing asemiconductor substrate having a first major surface and an insulatinglayer present on at least the first major surface and comprising atleast one layer comprised of aluminum oxide, titanium oxide, siliconnitride, SiN_(x):H, silicon oxide, or silicon oxide/titanium oxide; (b)applying a paste composition onto a preselected portion of theinsulating layer of the first major surface, wherein the pastecomposition comprises in admixture: i) a source of electricallyconductive metal; ii) a fusible material comprising: 70-90 wt. % Bi₂O₃,8-15 wt. % Al₂O₃, 2-26 wt. % SiO₂, 0-9 wt. % B₂O₃, the weightpercentages being based on the total fusible material, and wherein someor all of at least one of the oxides is optionally replaced by afluoride of the same cation in an amount such that the fusible materialcomprises at most 5 wt. % of fluorine, based on the total fusiblematerial; and iii) an organic vehicle, wherein the paste composition islead-free and comprises the fusible material in an amount that rangesfrom 0.5 to 5 wt. % of the paste composition; and (c) firing thesubstrate and paste composition thereon, whereby the electricallyconductive structure is formed on the substrate.
 2. The process of claim1, wherein the source of electrically conductive metal is silver powder.3. An article comprising a substrate and an electrically conductivestructure thereon, the article having been formed by the process ofclaim
 2. 4. The article of claim 3, wherein the substrate is a siliconwafer.
 5. The article of claim 4, wherein the article comprises aphotovoltaic cell.
 6. The article of claim 3, wherein the articlecomprises a semiconductor device.
 7. The process of claim 1, wherein theinsulating layer is comprised of silicon nitride or SiN_(x):H.
 8. Theprocess of claim 1, wherein the insulating layer is penetrated and theelectrically conductive metal is sintered during the firing, whereby anelectrical contact is formed between the electrically conductive metaland the substrate.
 9. A process for forming an electrically conductivestructure on a semiconductor substrate, the process comprising: (a)providing a substrate having a first major surface and an insulatinglayer present on at least the first major surface and comprising atleast one layer comprised of aluminum oxide, titanium oxide, siliconnitride, SiN_(x):H, silicon oxide, or silicon oxide/titanium oxide; (b)applying a paste composition onto a preselected portion of theinsulating layer of the first major surface, wherein the pastecomposition comprises in admixture: i) a source of electricallyconductive metal; ii) a fusible material comprising: 70-90 wt. % Bi₂O₃,6.5-15 wt. % Al₂O₃, 2-8 wt. % SiO₂, 0.5-9 wt. % B₂O₃, the weightpercentages being based on the total fusible material, and wherein someor all of at least one of the oxides is optionally replaced by afluoride of the same cation in an amount such that the fusible materialcomprises at most 5 wt. % of fluorine, based on the total fusiblematerial; and iii) an organic vehicle, wherein the paste composition islead-free and comprises the fusible material in an amount that rangesfrom 0.5 to 5 wt. % of the paste composition; and (c) firing thesubstrate and paste composition thereon, whereby the electricallyconductive structure is formed on the substrate.
 10. The process ofclaim 9, wherein the fusible material is zinc-free.